Are you sure that you arent hitting any power limits etc, i have just built a second rig and while i have an 850w PSU the voltage delivery is clearly crap (it was a cheap PSU). This means in my new rig it runs as you say anywhere from 15 mins to an hour (for me) then craps out. When i try to get it running again it will give me cuda errors etc, sometimes wont detect the card, other times i get 0 hash rate.
I know its not a simple thing to check or test without investing in another PSU unfortunately but if the card will overclock and will run for a period of time and you are sure its not overheating then the most lilkely cause is PSU issues (IMO). There are im sure 100 other suggestions but having built PC’s for over 20 years 9/10 times the simple solution is the most effective one.
Hope it helps, and hopefully you can source another PSU to at least test with before buying an upgraded one.